These teachings relate generally to scintillation materials, and, more particularly, to scintillation materials with reduced afterglow.
Scintillation detectors are used in a wide variety of applications ranging from medical imaging to high-energy physics (HEP). The development of new scintillators with better properties, complemented by advances in silicon based readout technologies such as high-gain avalanche photodiodes (APDs), have significantly advanced the present detector state-of-the-art. High resolution x-ray detectors based on amorphous silicon sensor arrays (a-Si:H) or charge coupled devices (CCD) in conjunction with scintillator films have been developed, and are now routinely used in such applications as chest radiography and digital mammography. The medical community is particularly interested in new fast scintillators with high density and light output for applications in nuclear medicine, single photon emission computed tomography (SPECT), positron emission tomography (PET), computed tomography (CT), and diagnostic x-ray imaging.
While a wide variety of new scintillators are now available, doped alkali halide scintillators have proven useful and practical. This is especially true of CsI(Tl), which is a highly desired material for a wide variety of medical and industrial applications due to its excellent properties, low cost, and easy availability. Having the highest conversion efficiency of any known scintillator (64,000 photons/MeV), a rapid initial decay (680 ns), an emission in the visible range (540 nm), and cubic structure that allows fabrication into micro-columnar films, CsI(Tl) has found extensive use in radiological imaging applications. Furthermore its high density (4.53 g/cm3), high atomic number (Z=54), and transparency to its own light have made it a material of choice for x-ray and gamma ray spectroscopy, homeland security applications, and nuclear medicine applications such as intra-operative surgical probes and SPECT.
Commercial applications such as CT require a very large number of scintillator elements, so that the cost and availability of the scintillation material and any necessary machining have become very important issues. For instance, the major advance in CT technology within the past few years has been the development of volumetric scanning capability using multiple ring detector systems that need as high as 10,000 detector elements per scanner, and over 2,000 such scanners are produced annually. The material cost of the CdWO4 scintillator, which is currently used in such systems, is ˜$ 40/cm3 compared to ˜$ 1.6/cm3 of the CsI(Tl). Another new exotic material, which is now being used in CT systems, is ceramic GOS, which is even more expensive. Thus, replacing CdWO4 or ceramic GOS with CsI(Tl) would result in substantial cost savings. Moreover, the machining of CdWO4 pixels to dimensions as small as 0.8×0.8 mm becomes particularly expensive process due to the brittle nature of the crystal, resulting in significant material loss during processing. By comparison, CsI(Tl) is much easier to process, so that replacement of CdWO4 with CsI(Tl) would result in significant savings in machining costs. Finally, high quality CsI(Tl) is widely available from a number of commercial vendors.
Another rapidly growing area is high-speed digital x-ray imaging. Such applications require imaging speeds ranging from 30 frames per second (fps) as in fluoroscopy to 106 fps for analyzing ballistic impacts. Due to the limitations in the availability of x-ray flux, all such applications require a scintillator with very high x-ray absorption and high light output. As such, CsI(Tl) can be a prime choice in such applications.
Despite the obvious advantages of CsI(Tl), a characteristic property that has hindered its use in CT and many other high speed imaging applications is the presence of a strong afterglow component in its scintillation decay. Although the initial decay of CsI(Tl) has a characteristic time of 680 ns, its residual afterglow at 2 ms after the excitation can be as high as 5% of the peak value, depending on the intensity and duration of the excitation pulse. This causes pulse pileup in high count rate applications, reconstruction artifacts in CT applications, and problems of reduced contrast and image blurring in high speed x-ray imaging.
CsI is a member of a family of halides that had been studied for decades. The scintillator aspects of some of these halides again came to the fore in the mid-nineties because of newly developed medical applications (CT, PET), and stimulated a great deal of research interest.
The archetype of all the alkali halide scintillators is NaI(Tl), which was discovered in 1948 and is still in some use today. Its companion alkali halide scintillators, KI(Tl) and KCl(Tl), have played a very important role in our understanding of scintillator phenomena but have not been considered as viable scintillators for practical use.
It is the presence of Tl+ as an activator (a scintillation activator) that is responsible for the excellent scintillation properties of the alkali halides. Pure (undoped) NaI is not a good scintillator, emitting only poorly at room temperature. At 78K, however, it shows an emission band around 300 nm; this is similar to most other alkali halides, whose emission spectra are typical examples of self-trapped exciton (Vk+e) emissions. All are strongly temperature quenched, making the decay time progressively shorter and reducing the efficiency of both radiative and excitonic transfer in most of these materials. Interestingly, CsI is a special case, in that it retains some luminescence even at room temperature. While thermal quenching keeps the quantum efficiency Q rather low, it also gives rise to a relatively fast (although non-exponential) decay of about 16 ns, making the material useful where speed is the primary concern.
Upon doping with Tl+, however, the situation is changed radically, and CsI(Tl) has now become a highly important scintillator, as did NaI before it. The Tl+ ion, substituting at an alkali metal site, is a well-known luminescent center, whose 5d106s2 configuration gives rise to a 1S0 ground state and excited states 1P1, 3P0, 3P1, 3P2. The luminescence takes place from the lowest lying 3P0 triplet and is therefore spin-forbidden. The initial decay is on the order of 0.6 μs, which excludes CsI(Tl) from PET but still leaves it fast enough for many other applications such as CT, mammography, and digital radiography.
The physical mechanism of scintillation in alkali halides is as follows: When the material absorbs ionizing radiation, its energy is deposited into the lattice in the form of electron hole pairs (e-h pairs). However, since concentration quenching typically limits the Tl doping level to about the 1000-ppm range in CsI(Tl), very few of these charge carriers are formed close enough to the Tl ions to excite them directly. Thus, in order to reach the emitting center, almost all of the carriers must migrate over a significant distance. It is this migration of charge carriers, and their radiative recombination via the Tl ion, that constitutes the principal mechanism of scintillator light generation in CsI(Tl).
A problem in all alkali halides is that the mobility of carriers is very small due to self-trapping. This is particularly true for holes, leading to the formation of Vk centers (I2-molecules), which can move only by a temperature-dependent diffusive mechanism. Additionally, since charge states Tl0 and Tl2+ are known to exist and have lifetimes longer than the radiative lifetime of the Tl+ emission, either holes or electrons can be trapped by the Tl+ ions, which then have to wait for the second carrier to arrive.
This is a primary reason for the complexity of the scintillation process in the alkali halides. Electrons and holes produced by ionizing radiation can both be trapped at thallium activators since both Tl0 and Tl2+ are stable in most alkali halides. Accordingly, recombination can proceed by a number of different routes. Moreover, holes may also be self-trapped in the lattice, forming small polarons called VK centers that diffuse by thermally activated reorientation. Such VK centers in CsI(Tl), created by prolonged x-irradiation at low temperature, were investigated by the technique of magnetic circular dichroism of absorption (MCDA), from which it was concluded that the Tl0+ ions serve as shallow hole traps, and consequently that persistent afterglow is unavoidable in this material.
The carrier-mediated transfer in Tl+-doped alkali halides is complex. As in all sequential processes, the overall rate is determined by the slowest step, which in the halide case is likely to be the motion of self-trapped holes (diffusing Vk centers) or the detrapping of electrons. Hole motion would be difficult to detect if not for the fortunate happenstance of a well-defined 300 nm absorption band due to the Tl2+ ion. The intensity and time dependence of this absorption provide all the information needed to define the hole capture kinetics. The contributions of the various processes manifest themselves at different times, and can also be readily affected by temperature. The resultant luminescent decay can be broken into three time domains detailed below.
1. Early Time Processes:
This contribution is largely due to the creation of an excited state (Tl+)*, such as by recombination between a free electron and a hole trapped at a Tl site. In both NaI(Tl) and KI(Tl) a Tl+ ion captures a hole located in any of 25 neighboring unit cells, before that hole has had a chance to self-trap. This process is temperature-independent and determines the initial decay rate of the pulse, with a time constant characteristic of the Tl+ ion (≈200 ns). The contribution of this component to the total light output is not very large, on the order of 10%. The hole motion is presumably a random walk from cell to cell, but rapid enough to allow it to reach the ion before becoming self-trapped. This process also plays an important role in some Ce-activated scintillators.
2. Intermediate Time Processes:
Here the major process is a thermally activated diffusion of Vk (self-trapped holes), allowing them to reach and recombine with electrons trapped at thallium sites)(Tl0). This is the regime where individual halides differ because of the wide disparity in diffusion times, ranging from 10−9 s for NaI, 10−7 s for KI to 10−2 s for KCl. Consequently, the diffusion-controlled long component is considerably less important in NaI(Tl).
3. Long Time Processes:
Here the dominant mechanism involves the prompt capture of electrons and holes at different thallium sites (forming Tl0 and Tl2+, respectively), followed by thermal detrapping of electrons from Tl0 and their subsequent capture by Tl2+ to form (Tl+)*. This is the origin of the very long component in halide scintillators, particularly evident in KI(Tl) where it contains 75% of the light output and lasts over 200 ms.
When utilized in radiation detectors, the cesium iodide (CsI) scintillator absorbs photons of a given energy and converts their energy into photons of a lower energy, the latter detectable by photodiodes or CCD arrays. These lower energy photons are guided towards a photodiode or CCD array where their energy is absorbed and used to generate charge or charge depletion. Cesium iodide (CsI) scintillators can have a single crystal structure or polycrystalline ceramic structure. In some applications, each of the photodiodes or CCD elements is a picture element (pixel) resulting in spatial sampling of the image, which is the first step in image digitization. In some detectors, the CsI scintillator is deposited directly on top of the photodiode array. In some applications, the CsI scintillator is grown in a multi-columnar structure (very thin needles) that channels the lower energy photons towards the photo-diode array. In other applications a lens, fiberoptic guide or air spaces may be used between the scintillator and the photodiode or CCD elements.